No employers of mainland visitors working illegally on two-way permits on Hong Kong construction sites have been prosecuted because of the difficulty in gathering evidence, Secretary for Security Regina Ip Lau Suk-yee said yesterday. Mrs Ip told Legco seven site bosses were arrested last year for hiring visitors but none was prosecuted. By contrast, 259 mainland visitors or illegal immigrants were arrested on construction sites. Only two site bosses were prosecuted for employing illegal immigrants with no papers. 'The problem is that we often lack sufficient evidence to press charges against bosses whom we suspect of employing mainland visitors on their sites,' Mrs Ip said. She said the problem of illegal mainland workers had been brought under control through stepped-up raids by police and the Immigration Department. In 1999, 785 mainlanders were arrested at construction sites, compared with 356 in 2000. There were 275 arrested in the first nine months of this year. The chairman of the Hong Kong Construction Industry Employees' General Union, Choi Chun-wa, criticised what he described as government inaction over site bosses. He said only by prosecuting bosses would the building industry get the message that illegal workers would not be tolerated. 'This is ridiculous. On one hand they tell people they have been making an effort to stem the number of illegal workers, while on the other hand they say there is nothing they can do about unscrupulous bosses,' Mr Choi said. He said although the sector had many layers of subcontractors, police and immigration officers could interrogate the illegal workers to find out who the bosses were. Mainland workers are paid about $300 a day, compared with $700 for local workers. Mr Choi said unscrupulous bosses avoided paying illegal workers by reporting them to immigration or police when they had finished the job. Mrs Ip also said more mainland women were being arrested for prostitution - 5,900 in the first nine months of this year. There were 5,228 arrests last year, 3,894 in 2000 and 2,715 in 1999.